Passive shippers typically use a phase change material (PCM) that changes from solid to liquid and vice versa (e.g., ice to water and water to ice), where the PCM's temperature changes minimally while the PCM absorbs or releases energy due to external/internal temperature differential. This is how ice is able to maintain a beverage cold, by absorbing heat from the beverage (which itself is absorbing heat from the environment or the user's hand) while the ice turns into a liquid (at zero degrees Celsius/thirty-two degrees Fahrenheit). This is also how commonly available water-based gel packs or packets are able to maintain temperatures near zero degrees Celsius inside of an insulated lunch box or camping cooler.
The term “passive” is used in “passive shippers” because these types of systems are only able to maintain one temperature (the phase change temperature, such as for example zero degrees Celsius) and only in one direction per phase change condition. In this way, a frozen block of ice can only maintain zero degrees Celsius and protect against a temperature differential that is above the phase change temperature of the material (zero degrees Celsius or above). For example, a frozen block of ice cannot maintain zero degrees Celsius when exposed to a temperature below zero degrees (e.g., negative twenty degrees Celsius). Using frozen ice to protect a product that cannot be exposed to, for example, negative twenty degrees Celsius may not be ideal because no phase change will occur in the ice from zero degrees Celsius to negative twenty degrees Celsius.
However, the same frozen block of ice can protect a product that must be at zero degrees Celsius against warm temperatures, for a time period, because melting occurs at zero degrees Celsius, and the temperature of zero degrees is maintained during the time period while the ice melts.
Typical “passive” systems are not able to adjust to outside temperatures in order to maintain the appropriate temperature range. For example, consider a product that needs to be maintained between negative ten degrees Celsius and ten degrees Celsius. If only frozen ice were used in a passive shipping system, then only protection over ten degrees Celsius could be provided, and for a certain amount of time (the time for the ice to melt). Thus, frozen ice may be effective in the summer, where an ambient temperature of thirty degrees Celsius would otherwise warm the product. However, the frozen ice would not protect against a negative ten-degree temperature.
An option to overcome this problem could be to combine frozen ice with liquid water in the same shipping container. Because both liquid water and frozen ice will equilibrate at zero degrees Celsius (thus no temperature differential, therefore no heat transfer and no change in temperature, for the time period while melting or freezing occurs) zero degrees Celsius can be maintained for a certain period of time in both winter and summer environmental conditions. This can be a very cost effective and efficient way of accurately maintaining zero degrees Celsius inside of a shipper. However, the fact that a frozen and refrigerated water shipper is excellent for zero degrees Celsius means that it may not be suitable for a range of two to eight degrees Celsius since this range is above or outside zero degrees Celsius.
Various types of passive shippers for shipping refrigerated products use PCMs, including water-based PCM gel packs or packets and custom PCM packs.
Water-based PCM gel packs or packets: An insulated shipper with a passive water-based PCM may be used to maintain a constant temperature inside of a payload chamber. The advantages of water-based PCM gel packs or packets are low cost, low toxicity, and minimal environmental impact (disposability). Water-based PCM gel packs can be easily gelled to prevent leakage in the event of a puncture and can make the gel pack more rigid. These gel packs are usually tested against standard temperature profiles that simulate, for example, twenty-four, forty-eight, seventy-two, or ninety-six hours environmental conditions for worst-case winter and summer conditions.
However, with this type of shipper, water changes phase at zero degrees Celsius (thirty-two degrees Fahrenheit), which may be too low for pharmaceutical products or other products and can lead to freezing of the product. Accordingly, a buffer component can be added between the zero-degrees Celsius frozen water-based gel pack and the product (which requires a temperature of, for example, two to eight degrees Celsius). Such buffer components can include, for example, refrigerated water-based gel packs, bubble wrap, an air gap to avoid freezing the product.
Custom PCM Packs: An insulated passive shipper with a passive custom PCM may be used to maintain a constant temperature inside of the payload chamber. A custom PCM is a chemical, other than plain water, which is chosen for its freeze and melt point to maintain a temperature other than zero degrees Celsius, the freeze and melt temperature of water. Custom PCM packs are advantageous in that they are less expensive than active shippers (e.g., compressor-driven systems, Peltier-based devices, heater/cooler devices, etc.). Custom PCM packs usually have a much lower (half or less) heat of fusion (amount of energy required to melt or freeze a quantity of mass of material, or how long the material will maintain a certain temperature or ‘last’) when compared to water. This means that there is much less energy involved in the freezing and melting process, and therefore it can take more mass of custom PCM (as compared to waster-based PCM), which in turn means that the overall scale of the shipper may be larger and heavier. Custom PCM packs typically control temperature for a shorter period than water-based PCM packs.